Encapsulation for an electrical component and method for producing the same

ABSTRACT

Encapsulating a component structure includes applying a fluid, light-sensitive first reactive resin layer to a surface of a component substrate containing the component structure, exposing and developing the first reactive resin layer so as to form a frame structure that encloses the component structure, covering the frame structure with an auxiliary foil, applying a second reactive resin layer to a surface of the auxiliary foil so as to form ceiling structures on the surface of the auxiliary foil, at least one of the ceiling structures making a seal with the frame structure, and removing the auxiliary foil in areas between ceiling structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/DE01/00404,filed on Feb. 2, 2001, and to German Patent Application No. 100 06449.6, filed on Feb. 14, 2000.

BACKGROUND

In the manufacture of electrical and electronic components from wafers,it is often necessary to provide sensitive component structures with acover while they are still on the wafer, i.e., prior to their beingseparated into individual components. This cover can, for one thing,serve as protection against external influences further along in themanufacturing process or can become the foundation for certain packagingtechnologies. A cover of this type can form a mechanical protectionagainst media introduced further along in the process or as a protectionagainst contamination, in particular against electrically conductiveparticles, which are particularly disruptive in miniaturized components.The packaging can be completed based on this cover, for example in aresin mold, compressing it in plastic, installing it in another housing,etc.

Surface acoustic wave devices have especially sensitive componentstructures that can include inter-digital transducers and reflectors inthe form of finely structured conduction paths. These sensitivestructures are not, in general, permitted to come into contact with thecovers noted above, since this contact can cause properties of thesurface acoustic wave devices to change in an unreliable andnon-reproducible manner. To cover such structures requires anencapsulation that forms a cavity over the structures.

From WO 95/30 276, an encapsulation for electrical and electroniccomponents is known that comprises a frame and a support structure thatencloses the component structures and a cover layer that sits on top ofthem that can form a hermetically sealed cap, together with the frame,that encloses the sensitive structures. It is preferred that both theframe structure as well as the cover layer be made of a photoresistmaterial, and particularly of a photoresist foil. These are laminatedover the entire surface, exposed via a mask, and developed. These stepsare separately carried out for the frame structure and the cover layer.Overall, the process is relatively intensive and requires a multitude ofprocess steps.

SUMMARY

The object of this invention is thus to provide a process to encapsulatesensitive component structures that is simpler to carry out, yet leadsto a secure encapsulation of even sensitive component structures.

Advantageous embodiments of the invention as well as an encapsulationthus produced can be found in the claims.

Advantageous embodiments of the invention as well as an encapsulationthus produced can be found in the claims.

Like the known encapsulation just described, the process according tothe invention is based on a two-part encapsulation system, namely aframe structure that encloses the component structures and a ceilingstructure that sits on top of it. According to the invention, however,the difference from the known process is that the reactive resin isapplied in fluid form both for the frame structures as well as for theceiling structures, said resin being cured afterward. After the framestructures are established by the exposure and development of a firstreactive resin layer, an auxiliary foil is stretched over the entiresurface, if necessary over the entire wafer, and not just over theindividual component. This foil covers all frame structures produced onthe substrate, and adheres to them. The second reactive resin layer isthen applied on top of the auxiliary foil in a structured manner. In thelast step, the areas of the auxiliary foil outside the frame structuresand between the ceiling structures are removed, preferably by dissolvingthem using a solvent or by treating them with a plasma.

In a preferred embodiment of the invention, a UV-cured reactive resin isused for the second reactive resin layer that is again structurallyexposed, developed and cured after being applied to the entire surface.

A special advantage is that it is very simple and controllable to spincoat the fluid reactive resin, and it is easier to execute than thelamination of resist foils. It is also more cost effective overall. Thesecond reactive resin layer provided for the ceiling structure can thenbe applied in any desired thickness to ensure sufficient mechanicalstability for the encapsulation. The application of further layers overthis ceiling structure is then not required.

In the process, the image exposure of the first and second reactiveresin layer can each be done using a laser, in particular using a UV(ultraviolet) laser. This is advantageous, since the process can beeasily adjusted according to varying component structures, eliminatingthe need to first produce expensive photo masks for the exposureprocess.

Advantageously, parallel to the manufacture of the frame structures inthe first reactive resin layer, other component structures can beproduced at the same time. Examples of such structures include dampingstructures in surface acoustic wave devices. In this case, it isadvantageous if the material of the reactive resin layer is matchedacoustically, i.e., having a suitable hardness and a suitable modulus ofelasticity.

It is preferred to develop the reactive resin layers using a fluiddeveloper. Depending on the resin system used, the fluid developer maybe an organic solvent or an aqueous-alkaline solvent.

In another embodiment of the invention, the second structured reactiveresin layer is produced by structured printing onto the auxiliary foilin the area over the frame structures and then curing. Suitablestructured printing processes include screen printing and stencilprinting. Thanks to the auxiliary foil, the printing is not criticaleven when the structural dimensions of the component structures aresmall, since it is only necessary to cover the frame structures with thesecond reactive resin layer. The printing precision of the print processnoted above is sufficient for this purpose even if the printing processis not suited for producing the first reactive resin layer.

This auxiliary foil can be a thin plastic foil, and in particular it canbe a thin thermoplastic foil. This is advantageous since it allows theauxiliary foil to be easily stretched, producing an even covering overthe frame structures. Furthermore, the plastic foil can connect to theframe structures easily, e.g., by adhesion, melting or fusing.

In the process, the material for the plastic foil can be selected suchthat it can be easily removed. The material may be soluble in a solventused to remove the plastic foil in the areas between the framestructures that are not for the purposes of encapsulation.

An appropriately thin plastic foil can also be easily removed in plasma,in particular in plasma containing oxygen. In the process, the thicknessof the second reactive resin layer is selected to provide sufficientexcess to provide for the dissolution that acts on this second reactiveresin layer as well during this removal process. Suitable materials forthe plastic foil include, e.g., polyamide, PET, or polycarbonate foils.In the process, the thickness of the plastic foil is selected so that onone hand it is as thin as possible, and on the other hand it issufficiently sturdy so as to support the second reactive resin layerwithout sagging too much. Usually, foil thicknesses near 1 μm aresufficient. However, thicker foils of up to approximately 20 μm can alsobe used, as can thinner foils. For example, the foil thickness can befrom 0.5 to 5 μm.

The following explains the invention in more detail with the help ofrepresentative examples and with the help of nine figures.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 8 show various process steps to produce theencapsulation of the present invention, shown by schematic crosssections through a component to be encapsulated.

FIG. 9 shows a plan view of the component during a stage in the process.

This embodiment is directed to the encapsulation of active, and thussensitive, component structures of a surface acoustic wave device. Theexplanatory figures are only schematic representations and thus are notto scale.

DETAILED DESCRIPTION

FIG. 1 shows a piezoelectric substrate 1, such as a wafer made oflithium tantalite or lithium niobate. Various component structures areapplied to the surface of the substrate 1. Examples of such structuresinclude electrically conducting structures such as an aluminum (e.g.,metal) layer. In FIG. 1, only sections of this metal layer are shown,namely the sensitive component structures 2, which can belong todifferent components although they are located on a single wafer. Alight-sensitive reactive resin layer 3 is spun onto the entire surfaceof the substrate 1 over the component structures. The resin layer 3 mayhave a thickness of approximately 50 μm. The thickness of the reactiveresin layer 3 is selected so that it exceeds the thickness of thecomponent structures 2 to a sufficient extent that a height differencebetween the top edge of the component structures 2 and the reactiveresin layer 3 ensures a safe distance for the cover of the componentstructures 2.

It is preferred to apply a cationically initiated UV-cured, solvent-freeepoxy resin as the reactive resin, cured by UV radiation. This reactiveresin contains another photo initiator in addition to the cationicallycured epoxy or a photo initiator system that matches the exposuresource. These types of epoxy resins are described in DE-A 44 43 946, forexample, which is incorporated herein by reference in its entirety. Tosupport curing, the reactive resin can contain additional additives,preferably of a basic nature and selected from the group of salthydroxides or organic amines.

Referring to FIG. 2, through scanned exposure 4 by a UV laser, certainareas 5 in the first reactive resin layer 3 are exposed and at leastpartially cured. The exposure causes a solubility gradient to occurbetween the exposed areas 5 and the unexposed areas of the firstreactive resin layer 3, allowing the development to be carried out usinga developer solution.

FIG. 3 shows the component after development. The exposed areas 6 of theoriginal first reactive resin layer 3 that remain after development formclosed frame structures 6 that enclose the sensitive componentstructures 2. The frame structures 6, if they are only partially cured,can be completely cured in this process step by increasing thetemperature for a short period. The complete cure can also occur priorto development.

A thin plastic foil 7 is thus stretched over the frame structures 6 suchthat the thin plastic foil sits on top of the frame structures 6. It ispreferable to stretch the thin plastic foil 7, which is used as anauxiliary foil, over the entire substrate 1.

FIG. 4 shows the arrangement resulting after foregoing process step. Byheating and layering the foil with adhesive, through laser or frictionalfusing or a similar means, the foil 7 is made to stick to the framestructures 6.

A fluid layer 8 of an equally light-sensitive reactive resin is thenspun onto the entire surface of the auxiliary foil 7. Preferably, thesame resin as was used for the first reactive resin layer is used here.The thickness of this second reactive resin layer 8 is selected suchthat it is considerably more than the thickness of the plastic foil 7.FIG. 5 shows the arrangement resulting after this process step.

A structured exposure is then performed on the second reactive resinlayer 8 to produce cured areas 9. The exposure is carried out in thesame manner as the exposure of the first reactive resin layer 3, e.g.,by scanned UV laser radiation. The cured areas 9 are arranged such thatthey form ceiling structures that fit over the frame structures 6,sealing them shut at their outer perimeter (see FIG. 6).

Referring to FIG. 7, if necessary, another temperature varying step isnow carried out to complete the curing in areas 9. Then, the unexposedareas of the second reactive resin layer 8 are removed using a fluiddeveloper, leaving the ceiling structures 10 behind. Depending on theresin system used, the developer can be an organic solvent or an aqueousalkaline solvent.

In the next step, the areas of the plastic foil 7 not covered by theceiling structures 10 are removed, for example, through short-termincineration treatment in suitable plasma, which may contain oxygen. Inthis etching step, the open, uncovered areas of the plastic foil 7 arecompletely removed. At the same time, some of the ceiling structure 10layer is removed. FIG. 8 shows the resulting completely encapsulatedcomponent structures 2. The encapsulation includes the frame structure 6and the ceiling structure 10 with auxiliary foil 11 residue in between.The component structures 2 are thus hermetically sealed and protectedagainst further aggressive process steps. It is also possible toseparate the components in this stage by subdividing the substrate 6between the component structures, i.e., the encapsulated components. Theseparation can be performed using a saw, for example.

FIG. 9 shows the arrangement of the frame structures 6 around thesensitive component structures 2. They are shown only schematically. Thesensitive component structures can be connected to electrical connectingsurfaces 12 on the substrate through conduction paths. The framestructure 6 is then arranged so as to enclose the sensitive componentstructures 2, but the connection surfaces 12 and, if necessary, part ofthe electrical leads to it sit outside the frame structure. In theprocess, part of the electrical lead is covered by the frame structure.After applying the ceiling structures 11, 10, the outer edges of whichcoincide with the perimeter of the frame structures 6, the electricalconnection surfaces 12 remain accessible and can be contactedexternally. For example, this can be done using flip chip bonding, inwhich the connection surfaces 12 are connected to a base plate usingso-called bumps. It is, however, also possible to connect to thecomponent via the connection surfaces 12 using bonding wires.

Further processing is preferred to be through flip chip bonding, withthe sensitive component structures sitting on the surface of thesubstrate that faces the base plate, and are thus additionallyprotected. By sealing the gaps between the substrate 1 and the baseplate as well (not shown in the figure), the component can be furthersealed.

Although the invention was described by means of one embodiment, it isnot limited to that embodiment. To the contrary, it is possible toencapsulate other components with the aid of the invention using othersubstrate materials, other reactive resins or another type of exposureas well. The type of component to be encapsulated, i.e., the componentstructures to be encapsulated, also determine the geometric dimension ofthe encapsulation, which can be widely varied.

1. A method of encapsulating a component structure, comprising thefollowing steps performed in order: applying a first reactive resinlayer to a surface of a component substrate containing the componentstructure, the first reactive resin layer comprising a light-sensitivefluid; exposing and developing the first reactive resin layer so as toform a frame structure that encloses the component structure; coveringthe frame structure with an auxiliary foil; applying a second reactiveresin layer comprising a fluid to a surface of the auxiliary foil so asto form ceiling structures on the surface of the auxiliary foil, atleast one of the ceiling structures making a seal with the framestructure; causing the second reactive resin layer to harden; removingthe auxiliary foil only in areas between ceiling structures.
 2. A methodaccording to claim 1, wherein applying the second reactive resin layercomprises: spin-coating the second reactive resin layer onto an entiresurface of the auxiliary foil, the second reactive resin layercomprising a light-sensitive fluid; and exposing and developing thesecond reactive resin layer so as to form the ceiling structures, atleast one of the ceiling structures forming a cavity with the framestructure.
 3. A method according to claim 2, wherein exposing at leastone of the first reactive resin layer and the second reactive resinlayer is performed using a laser.
 4. A method according to claim 2,wherein developing at least one of exposed first and second reactiveresin layers is performed using a fluid developer.
 5. A method accordingto claim 1, wherein the second reactive resin layer is applied in astructured manner to the surface of the auxiliary foil by directlyprinting ceiling structures onto the surface of the auxiliary foil.
 6. Amethod according to claim 1, wherein the ceiling structures are formedand then cured using one of a screen and stencil printing process.
 7. Amethod according to claim 1, wherein the auxiliary foil comprises thinplastic.
 8. A method according to claim 1, further comprising fixing theauxiliary foil to the frame structure using at least one of an adheringprocess and a fusing process.
 9. A method according to claim 1, whereinremoving is performed using an incineration treatment and plasma.
 10. Amethod according to claim 1, wherein removing is performed using asolvent.
 11. A method according to claim 1, further comprising formingdamping structures on the component substrate during exposing anddeveloping.
 12. A method according to claim 1, wherein the auxiliaryfoil comprises at least one of polyamide, PET and polycarbonate and hasa thickness of 0.5 to 5 μm.
 13. A method according to claim 1, whereinthe first and second reactive resin layers comprise a UV-cured epoxyresin.
 14. A method of encapsulating a wafer, comprising the followingsteps performed in order: applying a first reactive resin layer to asurface of the wafer, the first reactive resin layer comprising alight-sensitive fluid; exposing and developing the first reactive resinlayer so as to form a frame structure that encloses the wafer; coveringthe frame structure with an auxiliary foil, the auxiliary foilcomprising a thin plastic; applying a second reactive resin layercomprising a fluid to a surface of the auxiliary foil so as to formceiling structures on the surface of the auxiliary foil, at least one ofthe ceiling structures making a seal with the frame structure; hardeningthe second reactive resin layer; and removing the auxiliary foil only inareas between ceiling structures.
 15. A method according to claim 14,wherein applying the second reactive resin layer comprises: spin-coatingthe second reactive resin layer onto an entire surface of the auxiliaryfoil, the second reactive resin layer comprising a light-sensitivefluid; and exposing and developing the second reactive resin layer so asto form the ceiling structures, at least one of the ceiling structuresforming a cavity with the frame structure.
 16. A method according toclaim 14, wherein the ceiling structures are formed and then cured usingone of a screen and stencil printing process.